Microbes in Earth's Oceanic Crust May Gobble Oxygen

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Miles beneath the sea surface, buried beneath the seafloor
sediment, a relatively unstudied ecosystem of bacteria and other
microbes teems with activity in the Earth's oceanic crust. Some
scientists think this system could hold the largest reservoir of
life on Earth, but direct measurements from the
difficult-to-reach region remain scarce.

Now, an international team of researchers based at the Bigelow
Laboratory for Ocean Sciences in East Boothbay, Maine, have
developed a new model that they say offers the best estimates yet
for how much oxygen these
microbes may consume — a measure that could be used in
future studies to determine the size of this ecosystem.

"We know there's a vast reservoir of life in the ocean crust, but
unless we take steps to quantify its metabolism, we'll never know
how vast it is," study co-author Sam Hulme, of Moss Landing
Marine Laboratories in California, said in a statement.

The team collected new measurements from the seabed
— the
thick layer of mucky sediments that sits below the ocean
water but atop the rocks of the Earth's crust — in order to
infer what may be going on beneath these sediments, in the
oceanic crust. Their measurements showed that oxygen levels peak
near the seabed's borders with the water above and rocky crust
below, but dip between these two boundaries. This suggests that
oxygen travels into the sediments from above and below, but that
some process consumes oxygen within the sediment's thick middle
section. [ Extreme
Life on Earth: 8 Bizarre Creatures ]

Given the environmental conditions within the sediment, the
researchers said, microbes are the likely culprits for this
oxygen dip. Bacteria use oxygen to break down carbohydrates into
usable forms of energy.

The team used the seabed oxygen measurements to predict oxygen
flow into and out of the the crust
below the seabed. The model suggests microbes in the crust
likely consume significant volumes of oxygen, but work is still
needed to determine just how large the microbe community is based
on these measurements. The team details their findings today
(Sept. 27) in the journal Nature Communications.

If the
crustal microbial population is as abundant as some
scientists think it is, then it would comprise a significant but
currently overlooked component of Earth's carbon cycle, said
study co-author Beth Orcutt, of the Bigelow Lab. Further study of
these ecosystems will help clarify their role in the nutrient
cycles that control the abundance of life on Earth, she said.

In a broader sense, the microbes may also provide a window into
the earliest evolution of life on Earth, Orcutt said.

"If it turns out that there is a large reservoir of life in rocks
that we haven't accounted for, that might have interesting
implications for the evolution of life on Earth," Orcutt told
LiveScience. "If we have this reservoir of life that has genetic
novelties or new species, then that might help us understand how
life is connected on Earth, and how it is connected over geologic
time."

The team collected sediment cores along the
Mid-Atlantic Ridge, an underwater mountain range that bisects
the length of the Atlantic Ocean, nearly 3 miles (4.8 kilometers)
beneath the sea surface. The cores were collected in 32-foot
(10-meter) increments to make them easier to handle, and spanned
a total distance of up to 310 feet (94 m). The team used a
needlelike probe to measure oxygen levels throughout the
cores.

The researchers said they deployed sensors into the core holes in
the seafloor, and they plan to retrieve data from the sensors in
2014, to help verify the results of their model. They also plan
to more closely assess the seafloor sediment contents to
determine the species of microbes present and the food sources
available to the ecosystem.